“Are you up?” The emails arrive late, often after 1 a.m., tapped out on a secure BlackBerry from an email address known only to a few. The weary recipients know that once again, the boss has not yet gone to bed. The late-night interruptions from President Obama might be sharply worded questions about memos he has read. Sometimes they are taunts because the recipient’s sports team just lost. Last month it was a 12:30 a.m. email to Benjamin J. Rhodes, the deputy national security adviser, and Denis R. McDonough, the White House chief of staff, telling them he had finished reworking a speechwriter’s draft of presidential remarks for later that morning. Mr. Obama had spent three hours scrawling in longhand on a yellow legal pad an angry condemnation of Donald J. Trump’s response to the attack in Orlando, Fla., and told his aides they could pick up his rewrite at the White House usher’s office when they came in for work. Mr. Obama calls himself a “night guy,” and as president, he has come to consider the long, solitary hours after dark as essential as his time in the Oval Office.

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He works on speeches. He reads the stack of briefing papers delivered at 8 p.m. by the National Security Council staff secretary. He reads 10 letters from Americans chosen each day by his staff. “He is thoroughly predictable in having gone through every piece of paper that he gets,” said Tom Donilon, Mr. Obama’s national security adviser from 2010 to 2013. “You’ll come in in the morning, it will be there: questions, notes, decisions.” One night last June, Cody Keenan, the president’s chief speechwriter, had just returned home from work at 9 p.m. and ordered pizza when he heard from the president: “Can you come back tonight?” Mr. Keenan met the president in the usher’s office on the first floor of the residence, where the two worked until nearly 11 p.m. on the president’s eulogy for nine African-Americans fatally shot during Bible study at the Emanuel African Methodist Episcopal Church in Charleston, S.C. Three months earlier, Mr. Keenan had had to return to the White House when the president summoned him — at midnight — to go over changes to a speech Mr. Obama was to deliver in Selma, Ala., on the 50th anniversary of “Bloody Sunday,” when protesters were brutally beaten by the police on the Edmund Pettus Bridge. “There’s something about the night,” Mr. Keenan said, reflecting on his boss’s use of the time. “It’s smaller. It lets you think.”